The invention relates to a spraying booth and to a circulation system.
For coating paints and lacquers on large objects, particularly on cars, spraying procedures are mostly applied that are carried out in spraying booths. In order to draw overspray and, particularly, freed solvents off the interior of the spraying booth, a circulation flow of air through the booth is set going. In this way, high concentration of harmful substances in the interior can be avoided. Even in persons, who have to work within the booth, have to wear protective masks, it is laid down to supply the inner room or working room always with fresh air. Before being allowed to flow in, the fresh air or ambient air is brought up to a desired temperature. In winter or at low ambient temperature, the fresh air has to be heated from an ambient temperature of 0.degree. C., for example, up to about 20.degree. C., i.e. room temperature. In summer or at high ambient temperature, the fresh air would have to be cooled down to a desired room temperature.
The size of the spraying booths is adapted to the average dimensions of the objects to be treated in such a manner that there is some working space around the objects to be placed in a central area of the interior to enable the working staff to spray onto the object from all sides. Spraying booths for passenger cars have an area of a length of substantially at least 7 m and width of substantially at least 4 m. A car to be treated is placed through a short lateral wall formed as an entrance door in the direction of the longitudinal axis of the booth into the center of the interior. The car stands then on a grid below which an exhaust air filter is mounted. Below the exhaust air filter is an exhaust air channel through which exhaust air streaming through the filter unit can flow to an air propulsion device. In order to provide space for the exhaust air filter and the exhaust air channel below the bottom of the booth, known spraying booths need a fundament of about 60 cm in depth.
The ceiling of the booth comprises a grid on which a supply of fresh air filter is lying, and an air channel above it which carries air from the air propulsion device to the upper side of the fresh air filter. Air supply through the fresh air filter and exhausting air through the exhaust air filter have to be chosen in such a manner that air in the interior sinks down as much free of turbulances as possible. Turbulences or any deviation from air sinking down in a laminar fashion lead to prolonged times of dwell of partial quantities of air which can also be enriched in paint particles and solvent.
With substantially uniformly sinking air, sinking speeds in the order of 0.25 m/s are provided. For a booth having a cross-sectional area of 28 m.sup.2 (7 m.times.4 m), a throughput of air of 7 m.sup.3 /s or 25,000 m.sup.3 /h would result. In order to be able to heat such a large throughput quantity of air sufficiently, a heating device of high heating capacity, particularly of the order of 300 kW, has to be installed. For reducing the amount of heat required, plants have been built in which fresh air were mixed with up to 50% of exhaust air, and the air mixtures was subsequently introduced into the booth. The exhaust air has already the temperature desired and, thus, need not to be heated. By using mixed air, the proportion of solvents in the working area is increased.
A further known measure for reducing the amount of heat required consists in that part of heat energy of the exhaust air is transferred to the inflowing fresh air in a heat exchanger. It shows, however, that, even with the known approaches using heat exchangers, the power of heat required is still high enough that it has to be produced by a burner. The use of a burner results in high expenses for approval and investment, because a fuel supply, a safe combustion chamber and a smoke exhaust conduit or a chimney are necessary. An electrical heating line would require an electrical connection of a large line cross-section. Installing such a connection involves high costs, and it is even doubtful whether such a high electrical power would be approved at all. In addition, the costs of current to be expected lead to very high operating costs.